This invention is directed at an improved extraction zone tray and to an extraction zone comprising one or more of the subject trays. The present invention is of particular applicability in liquid-liquid extraction zones wherein a contaminant is removed from a feedstream.
Solvent extraction is well-known and has been used for many years for product separation. In the petroleum industry solvent extraction has been widely used for the removal of impurities from process streams, such as the removal of aromatic compounds from lube oil feedstocks. In liquid-liquid extraction one or more components in the liquid mixture are removed by intimate contact with another liquid which is selectively miscible either with the impurities or with the desired product. Liquid-liquid extractions may be carried out in a number of different ways, such as by batch, co-current or counter-current extraction. Countercurrent extraction frequently is a preferred method for effecting the extraction, since it is continuous and since fresh solvent typically contacts the product just before the product exits from the extraction zone. Usually the solvent utilized is selectively miscible with the impurity to be removed but not miscible, or only slightly miscible, with the product. Countercurrent solvent extraction techniques are widely used in the petroleum industry for effecting product purification. In the manufacture of lube oils, the lube oil feedstock frequently is passed through a countercurrent extraction zone to remove product impurities, such as undesired aromatic components. Solvents frequently employed for extracting the aromatic components from the lube oil feedstock include phenol and N-methyl pyrrolidone (NMP).
When it is necessary to increase the throughput of a lube extraction unit, for example because of increased product demand or because lower yields from a poorer crude require a higher feed rate, the solvent recovery sections of the plant often can be expanded by conventional means, such as more heat exchange, additional flash drums, larger pumps, or larger capacity control valves. The ability of the internals in the solvent contacting tower to handle the increased load then may become the limiting portion of the plant. Therefore, it would be desirable to provide internals for the countercurrent contacting which have higher capacity per unit of tower cross-sectional area than internals currently in use while retaining hydraulic stability and effective contacting for mass transfer. The design of such trays is complicated by the fact that several feedstocks of different density, viscosity and yield are often processed in the same extraction zone at different feed rates, temperatures and solvent treats. Moreover, for a given feedstock the flow rates vary considerably within the extraction zone from tray to tray, thus requiring a great deal of hydraulic flexibility for the trays. Furthermore, these systems are usually characterized by very low interfacial tension, so that while mixing and mass transfer are easy, subsequent separation of the phases by settling and coalescence is difficult. Thus, a problem encountered in the design of extraction zones is minimizing excess mixing to thereby avoid emulsion formation, excess recirculation and turbulence.
Another problem encountered in the design of extraction zones is the problem created by the reverse in the direction of the lighter phase as it flows upwardly through the extraction zone. Sharp reversals in the light phase flow path are believed to promote entrainment of the heavy phase with the light phase.
Yet another problem encountered in the design of extraction facilities has concerned the problem attendant with the scale-up of extraction zones. Frequently, as higher feed rates are proposed, the diameters of the extraction zones have been increased to maintain the same specific through-put in terms, for example, of barrels per day per square foot. Since the cross-section area available for fluid flow between the stages increases with the diameter, whereas the superficial cross-section of the extraction zone increases as the square of the diameter, a larger diameter extraction zone with the same specific through-put will involve a higher flow velocity between the trays than a smaller diameter extraction zone of the same tray spacing. It has been found that this higher flow velocity adversely affects extraction zone performance.
Yet another problem encountered in the design of extraction zones has been the necessity for installing support beams, such as I beams, beneath the tray decks for support especially in large diameter towers, since the normally flat sheet metal trays typically do not have sufficient strength to support personnel during construction and maintenance of the tower internals. These beams normally do not contribute to process performance. They must be carefully designed to permit access for maintenance of the tower internals while not affecting the tower hydraulics.
Still another problem encountered in extraction zone design has concerned tray fouling from sludge accumulation.
Previously, efforts have been made to improve the extractive process primarily by improving extraction tower internals. U.S. Pat. No. 3,899,299 discloses a countercurrent extraction zone in which the less dense feedstock enters at the bottom of the extraction zone, while the more dense solvent enters at the top. A series of horizontally disposed, vertically spaced-apart trays are located in the zone. The less dense feed rising through the column flows under the tray, over a dam-like device and passes into cascade weir means located at substantially the same elevation as the tray. The perforate plate of the weir means causes the feed to be broken into small droplets, which pass upwardly to the area beneath the next higher tray, where the droplets coalesce. This process of droplet formation and coalescence is repeated at each tray in the extraction zone. Simultaneously, solvent passes downwardly flowing generally across the top of each tray removing impurities from the droplets of feed rising through the solvent. It has been found that this design may not be completely satisfactory at relatively high feed rates because the build-up of oil under each tray, particularly the bottom tray, resulted in a loss of the lube oil which was entrained in the bottom extract stream.
U.S. Pat. No. 3,053,520 describes a gas-liquid separation zone in which a plurality of troughs are disposed on each tray of the extraction zone. Each trough has a cover with a serrated surface to distribute the gas evenly through the liquid at each tray. This design is not attractive for liquid-liquid extraction zones. In vapor-liquid separation, the density difference between the vapor and liquid is great so that a relatively quiescent settling zone is not necessary. By contrast, in liquid-liquid separation zones discrete mixing and settling zones must be provided.
U.S. Pat. No. 2,759,872 is directed at a liquid-liquid extraction zone in which each tray includes a rectangular riser having a series of partitions disposed beneath downcomers. The laminar flow from the rectangular risers is dispersed into droplets by the downwardly flowing heavy phase. This design is not desirable because the parallel baffles which form the riser and discharge channels must be very close together to achieve low velocity laminar flow by frictional resistance. These small channels are susceptible to plugging with dirt, scale and corrosion by-products. In addition, the velocity through the discharge baffles, which is necessary to provide the desired frictional resistance, may induce entrainment of the heavy phase in the light phase. Moreover, since restrictive orifices are not used to reduce pressure drop, the riser height above the tray must be relatively great for an effective hydraulic seal without excessive recirculation of the heavy phase.
It is desirable to provide an improved extraction tray for an extraction zone which minimizes stage-to-stage entrainment of the heavy phase with the light phase.
It also is desirable to provide an extraction tray with improved phase separation at high horizontal flow velocities.
It also is desirable to provide an improved extraction tray in which strength and support for the tray deck is incorporated into the tray design, rather than by the use of extraneous beams.
It also is desirable to provide an extraction zone which does not become easily fouled with sludge.
The present invention is directed at an improved extraction tray in which a channel means communicates with the tray deck. The channel means provides strength for the tray deck and also provides a fluid conduit for directing fluid from beneath the tray to a weir means associated with the tray.